Author

Abstract

Although it is evident that vitamin D plays an important role in bone health, there is some confusion over the role of vitamin D blood tests. The National Osteoporosis Society has produced a guideline for health professionals to help clarify the type of blood test that should be carried out, who should be tested, how to interpret the results and how and when to use vitamin D supplements.

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5 key points

Many patients are requesting vitamin D blood tests, which may not always be necessary

Vitamin D blood tests should only be given if the results are likely to affect clinical management

If supplements are prescribed, tablets or capsules are thought to be better absorbed than injections

There is a lack of evidence on what a “sufficient” level of vitamin D is for everyone

If a patient is thought to have low levels of vitamin D, supplements can be given without a blood test being carried out

Recently, there has been increasing interest in vitamin D and its potential for preventing a range of medical conditions. The role of vitamin D in the maintenance of bone health is well established (Box 1) but there is much debate within the scientific and medical community about the significance of low levels of vitamin D for bone health and how this should be managed in clinical practice.

Media coverage of these issues has led to an increase in public demand for vitamin D blood tests, which has financial and resource implications for the health service. In addition, there has been confusion, even among health professionals, about the role of vitamin D blood tests -which type of blood test is most reliable, who should be tested, how to interpret the results and how to use vitamin D supplementation appropriately.

The National Osteoporosis Society has responded by producing a guideline, developed by clinicians and scientists with expertise in osteoporosis and vitamin D, to help health professionals manage their patients. It provides practical advice based on their own views, experience and on current evidence.

Key recommendations

What test to use

The best way to measure vitamin D status is a blood test of serum 25 hydroxyvitamin D (250HD) (Prentice et al, 2008)

Who to test

Patients should only have a vitamin D blood test if the results are likely to affect clinical management, such as:

Patients with bone diseases (such as osteoporosis or osteomalacia) that may improve with vitamin D treatment;

Patients who are going to be prescribed specific osteoporosis drug treatments (denosumab and zoledronate) where vitamin D deficiency needs to be corrected to prevent patients from developing hypocalcaemia;

Patients with musculoskeletal symptoms that could be attributed to vitamin D deficiency. This would usually be generalised pain, which could be caused by osteomalacia (Atherton et al, 2009).

Who does not need a test

Routine vitamin D testing is usually unnecessary in patients with osteoporosis or fragility fractures who are being prescribed vitamin D supplements along with osteoporosis drug treatment.

Although people obtain small amounts of vitamin D from the diet, most vitamin D is produced by the body due to the action of sunlight on the skin.

Generally, if a patient cannot get enough calcium in their diet and is at risk of vitamin D deficiency, calcium and vitamin D tablets should be prescribed without the need for a blood test. Other patients at risk of vitamin D deficiency do not need a blood test before being prescribed vitamin D supplements (Box 2).

How to interpret vitamin D blood tests

Without evidence showing that there is a “sufficient” vitamin D level for everyone, the guideline reflects recommendations from the US (Holick et al, 2011; Institute of Medicine, 2011):

Serum 25OHD <30nmol/L is “deficient”;

Serum 25OHD of 30-50nmol/L “may be inadequate” in some people;

Serum 25OHD >50 nmol/L is “sufficient” for almost the whole population.

Who to treat with vitamin supplements after the blood test

Those in the middle range (serum 25OHD 30-50nmol/L) need a supplement if they fall into one of the following groups:

Fragility fracture, documented osteoporosis or high fracture risk;

Treatment with antiresorptive medication for bone disease;

Symptoms suggestive of vitamin D deficiency;

Increased risk of developing vitamin D deficiency in the future because of exposure to sunlight, religious dress, dark skin;

Raised parathyroid hormone levels;

Medication with antiepileptic drugs or oral glucocorticoids;

Conditions associated with malabsorption.

For anyone else in this range, there is insufficient evidence of any benefit from taking vitamin D supplements and patients should be given lifestyle advice on maintaining adequate vitamin D levels through safe sunlight exposure and diet.

Those with adequate levels (serum 25OHD >50 nmol/L) can be reassured and given lifestyle advice as above.

Vitamin D3 is thought to be more effective than vitamin D2 (Tripkovic et al, 2012) but D2 may be the only acceptable form for vegans. Tablets or capsules are preferable to injections as there is some evidence they are better absorbed (Nugent et al, 2009).

What doses of vitamin D supplement are appropriate?

If patients need vitamin D levels to be corrected quickly, a “fixed loading” dose can be given once, followed by “maintenance” therapy.

The following doses are recommended:

A loading regimen to provide a total of approximately 300,000 IU vitamin D, given either as separate weekly or daily doses over six to 10 weeks;

Maintenance therapy comprising vitamin D in doses equivalent to 800-2000 IU daily (occasionally up to 4,000 IU daily), given either daily or intermittently at higher doses.

There is no recommendation to use higher doses where there is no urgency.

Follow up blood tests

A month after a loading dose of vitamin D has been given, a calcium blood test is recommended to make sure the patient does not have primary hyperparathyroidism (Hannan et al, 2004).

Routine follow-up vitamin D blood tests are generally unnecessary unless vitamin D deficiency symptoms continue, a patient has malabsorption problems or if it is likely they have not been taking the prescribed supplements.

Conclusion

If a patient is likely to be deficient in vitamin D, taking supplements is a sensible preventive measure and they do not necessarily need a blood test. However, in specific situations, testing will be useful to plan treatment.

● Thanks to the authors of the National Osteoporosis Society Vitamin D guideline for permission to reproduce its contents for this article.